1. Field of the Invention
The present invention relates to a brushless motor, and more specifically, to a structure for suppressing generation of torque ripple, and for enhancing the efficiency.
2. Description of the Related Art
Power steering for appropriately assisting a force required at the time of steering operation becomes prevalent, and it becomes possible for a driver to carry out the steering operation with a slight force, and driving comfort and safety of a vehicle have largely been enhanced. There are electric power steering and hydraulic power steering having different power sources. The electric power steering is superior in efficiency because appropriate amount of assist force is generated when necessary. As a power steering motor for generating the assist force, a synchronous brushless motor such as a three-phase brushless motor is used.
The brushless motor comprises a rotor having a rotatable shaft as its axis, and a stator surrounding an outer periphery of the rotor. On an outer periphery of the columnar rotor, a plurality of field magnets are radially arranged in an axial stripe shape such that the south pole and the north pole appear alternately in the circumferential direction. As the producing method, a method in which a segment magnet is pasted on a surface of the rotor, a method in which a segment magnet is embedded in the rotor, or a method in which an annular magnet polarized with the south pole and the north pole alternately in the circumferential direction is fitted over the rotor is used. Axial stripe slots are formed in phase positions in an inner periphery of an annular stator core constituting the stator, and portions (teeth) between the slots are arranged radially and projecting toward an inner diameter direction of a stator yoke. Coils are wound around the teeth to form winding (armature winding) of each phase.
If three phase motor current which is changed into sine wave shape or rectangular wave shape (including trapezoidal wave shape) is supplied to the winding of each phase, the rotor rotates and the brushless motor is driven.
A phenomenon in which torque pulses is generated due to heterogeneity of interaction generated between magnetic field of a magnet and teeth or slot of the stator. This torque is called cogging torque. To suppress the cogging torque as small as possible, a magnetic pole shape of the rotor and teeth shape of the stator of the brushless motor are researched. For example, there is a known skew processing in which an axial stripe magnetic pole pattern of the rotor is inclined in the axial direction, thereby averaging torque generated every rotation phase, and smoothening the entire rotation torque. The induction voltage waveform of each phase can be formed into a smooth sine waveform by carrying out the skew processing.
When the brushless motor is used as the power steering motor and the steering mechanism is driven directly by the motor, it is necessary to reduce rotation variations of the motor, reduce noise, enhances torque generation efficiency, to reduce the motor in size and to enhance its performance so that the driver is not offended. This is strongly required when it is used in a heavy and large so-called luxury car class.
Usually, a brushless motor is driven by current which is varied into sine wave form with respect to time (sine wave drive, hereinafter) in many cases. In this case, motor current of each phase is in the sine waveform as shown in solid lines iu, iv and iw in FIG. 14, induction voltage (also called counterelectromotive force) is sine waveform having the same phase as the motor current as shown in solid lines eu, ev and ew in FIG. 15.
Generation torque of the motor is obtained by the product of the torque constant and the motor current. Since the torque constant corresponds to the counterelectromotive force constant, torque generated by each motor phase becomes substantially equal to the product of the induction voltage and the motor current, and in the case of the motor, the torque becomes equal to the product of the induction voltage of sine wave and current of sine wave.
A total value of generation torques of the phases, i.e., the generation torques of the entire motor is stable torque value having small torque ripple as shown in FIG. 16, but its torque value is small. This is because that since not only the motor current but also the induction voltage is also varied with time in synchronism with in the form of sine wave, the induction voltage cannot be utilized sufficiently for generating torque. In the conventional brushless motor of this kind, since the generation efficiency of torque is poor, there is limitation for reducing the motor in size and to enhancing the output of the motor.
To enhance the utilizing efficiency of motor current and induction voltage, the waveforms of the motor current and induction voltage should be increased flatly. A driving method (rectangular wave driving or trapezoidal wave driving, hereinafter) in which time variation waveform of motor current rectangular wave or trapezoidal wave shape, the induction voltage is adjusted to rectangular wave having the same phase, torque generated in each phase becomes equal to the product of induction voltage of the rectangular wave and current of rectangular wave. However, in the case of this method, since the motor current is rectangular waveform, the ON-OFF switching of current is abruptly changed, and there is inconvenience that variation is generated in motor torque, and it is necessary to take measures against this. It is difficult to generate the rectangular wave induction voltage having the same phase as that of the motor current only by devising concerning the motor structure and thus, it is necessary to adjust the driving control method and winding method, and it is difficult to produce a small and efficient brushless motor.
If the applied voltage of each phase is increased, the motor output is increased. That is, if input voltage can be increased, it is possible to reduce the size of motor which outputs predetermined torque. However, in the power steering motor for vehicle, since the applied voltage is determined by specification on the side of power supply, it is difficult to increase the applied voltage. Therefore, it is necessary to consider how to reduce the motor in size and to increase its output under the given applied voltage condition.